The present invention relates generally to an injection mold for forming a molded product of a synthetic resin and more particularly to such an injection mold which eliminates defects in the appearance of the molded product such as flow marks formed on those portions of the molded product which are disposed adjacent to the gates of the injection mold during the molding operation.
A audio or video magnetic tape cassette generally is formed of a case body composed of upper and lower cassette halves and a pair of hubs around which a magnetic tape is wound rotatably housed in the case body.
Generally, the upper and lower cassette halves are injection molded of a plastics material. More specifically a molten material is injected into a mold cavity via a gate, such as a direct gate and a submarine gate, so as to mold each cassette half, the cavity being formed by two mold members and corresponding in shape to the cassette half. The molding material filled in the cavity is cooled and is set or solidified to form the molded cassette half. This molded cassette half is removed from the mold, and thereafter for the purpose of achieving a desired design effect, a paper sheet or the like of a predetermined design is bonded to the outer surface of the cassette half. Alternatively a predetermined design can be printed directly on the cassette half.
The upper and lower cassette halves are commonly formed entirely of a light-transmitting material so that the winding condition of the magnetic tape can be readily observed from the exterior. Alternatively, as is the case of an audio compact cassette as shown in FIG. 4, window portions 6a and 6b of a transparent or a translucent nature are provided on a cassette half body 2a. The window portions 6a and 6b are formed in predetermined respective shapes for the purpose of achieving a desired design effect for the case body 1 and for other purposes.
Conventionally the window portions 6a and 6b are formed by bonding window members (made separately from the cassette half body 2) to the cassette half body 2 by an adhesive, ultrasonic welding or the like. In this case, however, since the step of shaping the window members and the bonding step are required, the design is limited and the productivity in manufacturing the cassette is not high. For these reasons, recently, a two-color molding method has been extensively used.
At present, the method which is most commonly used for two-color molding cassette halves is what is termed the "core back" method.
The process of manufacturing a cassette half by the core back method will now be described with reference to FIGS. 5 and 6.
Each of FIGS. 5 and 6 is a cross-sectional view of a cassette-half forming mold taken along a dot-and-dash line A--A in FIG. 4 showing an audio compact cassette.
As shown in FIG. 5, the injection mold for the core back method is composed of a fixed mold member 30, a movable mold member 31, and slide cores 33 and 34. A mold cavity 35 for molding the cassette half 3a, 3b is formed between the opposed surfaces of the fixed mold member 30 and the movable mold member 31. The slide cores 33 and 34 are movably received in the mold cavity 35 so as to form the window portions 6a and 6b.
A submarine gate 36 communicates with the mold cavity 35 so as to inject an opaque resin thereinto, and also direct gates 39 and 40 communicate with the mold cavity 35 so as to inject a transparent resin thereinto.
The fixed mold member 30 and the movable mold member 31 are mated with each other, and then the slide cores 33 and 34 are held against the inner surface of the fixed mold member 30 by a hydraulic drive device (not shown). In this condition, the opaque resin is injected into the mold cavity 35 through the submarine gate 36 to thereby injection-mold the cassette half body 2a. Thereafter as shown in FIG. 5, the slide cores 33 and 34 are suitably retracted to provide window-forming cavities 37 and 38. Then, the transparent resin is injected into the window-forming cavities 37 and 38 through the direct gates 39 and 40 which are provided in registry with the cavities 37 and 38, respectively thereby forming the transparent window portions 6a and 6b. The cassette half body 2a and the window portions 6a and 6b are fused together and formed into a unitary molded construction by the heat generated during the injection molding and by the injection pressure.
When the molten resin is injected into the mold cavity particularly through the direct gates 39 and 40 so as to form the window portions 6a and 6b, a shearing heat develops at the distal end of the direct gate. This shearing heat decomposes part of the injection resin to produce gas or causes refuse of the resin to deposit on the distal end portion of the direct gate 39, 40. As a result, turbulence develops in the flow of the resin so that appearance defects such as flow marks, are formed on those portions 41 of the molded product disposed adjacent the direct gates. Moreover unlike the submarine gate, in many cases the direct gates are unavoidably disposed in such positions as to form the outer surface portions of the case body, such as the window portions 6a and 6b, and therefore the such appearance defects markedly adversely affect the appearance of the cassette.